Marine Habitat Transitions and Body-Shape Evolution in Lizardfishes and Their Allies (Aulopiformes)

In this study, we use a geometric morphometric and a character evolution approach to study the evolutionary patterns of body-shape change and habitat transition in the Aulopiformes. Aulopiform fishes (lizardfishes; 289 spp.) inhabit diverse marine habitats from coral reefs to the deep sea and exhibit a wide range of body morphologies. Herein, we examine over 400 aulopiform specimens representing 38 of 44 genera and all families and identify that there are distinct patterns of body-shape change across the aulopiform radiation that coincide with habitat. A fusiform (torpedo-shaped) body is predominant among aulopiforms distributed in inshore-benthic and deep-sea benthic environments (e.g., Aulopidae, Bathysauridae, Synodontidae). There is a trend towards body elongation in taxa distributed in deep-sea pelagic habitats at depths of 200–4,000 meters (e.g., Alepisauridae, Lestidiidae, Notosudidae, Paralepididae) and a trend of body elongation with more centrally positioned dorsal and anal fins in the deep-benthic family Ipnopidae (tripodfishes). Additionally, deep-sea pelagic aulopiforms exhibit the largest variance in body-shape disparity with significant shape disparity compared to aulopiforms found in inshore-benthic and deep-sea environments. Deep-sea benthic lineages also have significantly higher body-shape variance and disparity compared to inshore-benthic lineages. We identify that there are considerable changes in body shape as aulopiform lineages transitioned to differing marine habitats. We infer the common ancestor of aulopiforms to have lived in a deep-sea benthic environment with a single transition to an inshore-benthic environment in the common ancestor of the Aulopoidei (lizardfishes, flagfin fishes) and two independent transitions into deep-sea pelagic environments, once in the common ancestor of Giganturidae, and once in the common ancestor of Alepisauroidea + Notosudoidea. This is the first study to quantitatively investigate changes in the body shape of aulopiform fishes tied to habitat transitions in marine environments from the deep sea to coral reefs. Our findings suggest that aulopiform body plans have broadly diversified in deep-sea pelagic and benthic habitats while remaining comparatively conservative in inshore-benthic habitats.